When mounting components such as flat package type IC, for example, QFP (Quadrate Flat Package) on a printed wiring pattern of a substrate, every connection between each pattern and its corresponding lead terminal is typically soldered in the following way: there are a number of the tips of lead terminals like crank legs protruding outward from the peripheries of ICs, and also there are formed a plurality of rectangular patterns (pads), the locations of which are preliminarily so determined that the patterns correspond to their lead terminals respectively, and lead terminals are placed on their corresponding patterns (pads). As a soldering method, a proper amount of soldering cream is printed and performed, a lead terminal is placed on the soldering cream, and heated to melt the soldering cream, and later cooled to be solidified. This reflow method is mostly practiced.
However, there are various causes such as variation in the length of a lead terminal protruding downward and poor contact between the lead terminal and a pattern at the time of soldering so that they may make solder solidified in such a situation that the lead terminal is remote from the pattern or the solder after solidifying.
Namely, FIG. 7(a) and (b) are illustrative views for comparing the cases between appropriate soldering and poor soldering and top views of mounting practice. The soldering cream printed on a pattern to is molten by heating thereof, and molten solder moves toward a lead terminal 4 due to capillary attraction. Thus, the molten solder is cooled to be solidified. When each lead terminal 4 of an IC 3 is connected by solder 5 to each pattern 2 on a substrate 1, in the Figure the lead terminal 4 on the right side has a good contact and is soldered well on the pattern 2. Whereas, if the lead terminal 4 does not contact with the pattern 2 before the soldering cream is molted by heat, the molten solder does not move toward the lead terminal 4 because capillary attraction caused by the lead terminal 4 does not affect the molten solder 5. Therefore, the molten solder 5 is solidified on the pattern 2 with uniform thickness. Accordingly, the lead terminal 4 on the left side is completely separated by not only the pattern 2 but also the solder 5.
In fact, there are intermediate cases between these extremes such that both of them are poorly soldered so that these insufficient soldered connections may be often the potential cause of making the lead terminal separated later.
Previously, there has been the difficulty of finding the case of such a poorly soldered connection. That is, formerly poorly soldered connections have been checked by visual inspection from the side of a substrate, and when the poorly soldered connection has been found, then it has been resoldered by soldering iron, resulting in soldering being completed as such. But it is very difficult to detect precisely such a case as being separated and poorly soldered connecting only by observing visually. In particular, it is impossible detecting the cases where both of them are just only slightly separated and no or extremely insufficient soldered connection is still formed though there could seem certain contact.
Since an electronic device cannot work naturally because of missing detecting a poorly soldered connection, the checking operation is critical and required to be perfect with no missing. However, as described above, missing will always take place due to such possible reason and it may happen that the electronic device does not work well after delivery as commercial goods.
Therefore, it is an object of the present invention to provide a printed wiring pattern in which there can be easily detected poorly soldered connection between the printed pattern and the lead terminal of an IC with visual inspection or an automatic checking method.